A comparison of the circular dichroism spectra of synthetic DNA sequences of the homopurine . homopyrimidine and mixed purine- pyrimidine types.

We have obtained the ultraviolet circular dichroism spectra of two repeating trinucleotide DNAs, poly [d(A-G-G).d(C-C-T)] and poly[d(A-A-G).d(C-T-T)], that have all purines on one strand and all pyrimidines on the other. These spectra, together with spectra of other synthetic polymers, can be combined to give 3 first-neighbor calculations of the spectrum of poly[d(A).d(T)] and 2 first-neighbor calculations of the spectrum of poly [d(G).d(C)]. The results show (1) that first-neighbor calculations utilizing only spectra of homopurine.homopyrimidine DNA sequences are no more accurate than are similar calculations that involve spectra of mixed purine-pyrimidine sequences, demonstrating that double-stranded homopurine.homopyrimidine sequences do not obviously belong to a special class of secondary conformations, and (2) that the wavelength region above 250 nm in the CD spectra of synthetic DNAs is least predictable from first-neighbor equations, probably because this region is especially sensitive to sequence-dependent conformational differences.